**Core Concept**
The development of external genitalia in a fetus is influenced by the balance of androgens and estrogens, which are crucial for the formation and differentiation of male and female external genitalia, respectively. The sex of the fetus is determined by the presence of the Y chromosome, which carries the SRY gene that triggers testicular development and testosterone production.

**Why the Correct Answer is Right**
The correct answer marks the stage at which the external genitalia of a fetus become clearly distinguished as male or female. This occurs around 8-10 weeks of gestation, when the effects of testosterone on the urogenital sinus and genital tubercle become apparent. The presence of testosterone causes the formation of a penis and scrotum in males, while the absence of testosterone or the presence of estrogens leads to the development of a clitoris and labia in females. The SRY gene and testosterone produced by the fetal testes are responsible for this differentiation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a clear stage in fetal development when the sex of the external genitalia becomes distinguishable.
**Option B:** This option is incorrect because it refers to a later stage in fetal development, when the external genitalia have already begun to take on a more definitive form.
**Option C:** This option is incorrect because it is not a specific stage in fetal development, but rather a general term that could refer to various periods of growth and differentiation.

**Clinical Pearl / High-Yield Fact**
The sex of the fetus can be determined with reasonable accuracy by examining the external genitalia at around 8-10 weeks of gestation, but it's essential to note that some cases of intersex or ambiguous genitalia may require further evaluation and diagnosis.

**Correct Answer: C. 8-10 weeks of gestation.**